US3416918A - Process of making dispersion strengthened lead - Google Patents

Description

United States Patent Office 3,416,918 Patented Dec. 17, 1968 3,416,918 PROCESS OF MAKING DISPERSION STRENGTHENED LEAD David Henry Roberts, Harlow, England, assignor, by mesne assignments, to St. Joseph Lead Company, New York, N.Y., a corporation of New York No Drawing. Filed May 12, 1966, Ser. No. 549,449 Claims priority, application Great Britain, May 19, 1965, 21,225/65 11 Claims. (Cl. 75-206) ABSTRACT OF THE DISCLOSURE The present application discloses a process for producing dispersion strengthened lead by mixing a small amount of a refractory oxide powder, such as alumina, with lead powder in an oxidizing atmosphere so as to coat the lead particles with lead oxide and thereby inhibit agglomeration thereof, while obtaining a substantially uniform dispersion of the refractory oxide. After said mixing process, the mixture may be subjected to chemical reduction of the lead oxide preferentially, while avoiding significant reduction of the refractory oxide and the so reduced powder mixture may be subjected to a forming process to amalgamate the lead particles into a coherent matrix with the refractory oxide dispersed throughout.

This invention relates to the production of dispersion strengthened lead and of lead powder from which the same can be made.

If lead powder having its constituent particles coated with lead oxide is subjected to a forming process which subjects the powder particles to shear deformation so that their oxide coatings become fragmented and dispersed, a coherent dispersion strengthened lead product can be obtained which exhibits considerably increased strength as compared with ordinary lead. A particularly effective forming process is extrusion, carried out at room temperature or at some higher temperature below the melting point of the lead in the powder.

Owing to the ease with which the lead oxide can be formed on the powder particles and can be subsequently dispersed in the forming process, the production of dispersion strengthened lead in this manner is relatively simple and economic. However the presence of the free lead oxide, which is not a refractory material in the generally accepted sense, tends to impart some susceptibility to corrosion, so that in this respect the product may be less good than ordinary lead. Moreover, the oxide coatings in general have a certain amount of lead carbonate associated with them and this can result in a tendency for the product to swell (due to liberation of carbon dioxide from the carbonate) at elevated temperatures.

Dispersion strengthening of various metals by the incorporation of dispersed refractory oxides such as alumina, silica and magnesia is a well known technique and attempts have been made to use it for dispersion strengthening of lead by mixing such refractory oxide with lead powder prior to extrusion or other forming process. However these attempts have met with only limited success, in part due to the difficulty of mixing the lead powder and refractory oxide to a satisfactory degree of dispersion while avoiding premature agglomeraiton of the lead particles due to their inherent softness and pliability.

According to the present invention a process for the production of dispersion strengthened lead comprises the steps of intimately mixing lead powder and a minor proportion of a refractory oxide powder in an oxidizing,

preferably oxygen, atmosphere such as to coat the lead particles with lead oxide and thereby inhibit agglomeration thereof by a pressure welding effect while obtaining a substantially uniform dispersion of the refractory oxide, subjecting the mixture to a reduction process for chemically reducing the majority of the lead oxide preferentially with respect to the refractory oxide by reason of the greater stability of the latter, and subjecting the so reduced powder mixture to a forming process to amalgamate the lead particles into a coherent matrix with the refractory oxide dispersed throughout. As the reduction process must be performed below the melting point of lead (so as to retain the powder form) reduction of all the lead oxide and simultaneously of lead carbonate tending to be present with it is not readily practicable. However the presence of only a small amount of lead oxide and lead carbonate in the powder mixture when subjected to the forming process and therefore in the final product means that the susceptibility of the final product to corrosion and swelling may be correspondingly small.

In carrying out the invention it is contemplated to use a refractory oxide powder consisting of particles of submicron size, while the lead powder particles may in the majority have an average size of about 50 microns or less. Thus a ratio of about *:1 between the average lead particle size and average refractory oxide particle size has been found to give good results but this is not limiting and better results may be obtainable with a different ratio, especially if lead powder of significantly smaller particle size becomes readily obtainable.

It is further contemplated in performing the invention to mix the lead powder and refractory oxide powder by ball-milling in preference to other mixing process such as vibration mixing and high speed blending. With ballmilling, in which the lead particles would be particularly prone to amalgamate if it were not for the presence of the oxidizing atmosphere, the charge of balls in the mill subjects the oxide coated lead particles to severe deformation and in doing so punches the alumina particles into the lead, thus producing a particularly homogeneous mix from which an extruded product with a more uniform refractory oxide distribution is obtained.

This preference for ball-milling as the mixing process is based on experiment by which this process was compared with vibration mixing and high speed blending. For these and the other experiments recorded herein, lead powder atomised from lead of 99.97% purity and having a nominal particle size of 300 mesh (that is, less than 53 microns) was used. However size analysis of the powder revealed that about 60 wt. percent of the particles were between 30 and 50 microns. The powder as received had a lead oxide content of about 1.35 wt. percent calculated at PbO. The refractory oxide powder used was alumina with a quoted particle size in the range of 03030-- 0.005 micron.

(Attempts to comminute alumina powder of this particle size by ball milling, with a view to obtaining a smaller particle size which might have had a greater dispersion strengthening effect, were unsuccessful and in fact resulted in lower strengths of the final lead product. It appeared that agglomeration rather than comminution of the alumina particles had occured, possible due to the build up of electrostatic charges on the insulating particles.)

For the comparison of the different mixing processes an addition of 2 vol. percent of the alumina powder to the lead powder was used.

For the vibration mixing a vibratory ball mill with stainless steel container was used without its balls. Charges of 298 grams of lead powder and 2 grams of alumina powder were vibrated at 1450 cycles per minute for varying times between 1 and 8 hours.

For the ball milling mixing process the same ball mill was used with 100 stainless balls of /2 inch diameter. Similar powder charges (corresponding to a 1:1 ratio of the gross volumes of the balls and powder respectively) and the same vibration frequency were used as for the vibration mixing, with varying milling times between /2 and 8 hours.

For the high speed blender a machine having six stainless steel blades rotating at 10,000 revolutions per minute in a tall glass container was charged with 894 grams of lead powder and 6 grams of alumina, this greater charge being necessary in order to cover the blades. The machine was run for a maximum of 3 minutes at any one time in order to prevent excessive oxidation of the lead powder due to overheating. The total blending'times were varied between 10 and 40 mins.

The mixed powders were all subjected to chemical reduction by hydrogen in order to eliminate the majority of the lead oxide originally present on the lead powder and subsequently produced during the mixing process. Experiments had shown that extensive lead oxide reduction could be achieved by subjection of oxidised lead powder at about 300 C. to a stream of purified dry hydrogen for 24 hours. Consequently the powder mixtures were given this treatment.

The reduced powder mixtures were in each case cooled to room temperature in hydrogen and then rapidly transferred (so as to avoid re-oxidation of the lead) to an extrusion press by which the subsequent forming process was to be performed.

A charge of about 100 grams of powder was required to fill the extrusion press. The powder was compacted and extruded in a single direct operation to produce a wire of 0.100 inch diameter. The extrusion ratio was 40:1 and no lubrication was used. The extrusion was performed at a rate of about 11% feet per minute at room temperature, but the temperature of the extrudate was about 60 C. because of the severe deformation involved.

A selection of specimen was chemically analysed to determine the quantity of residual lead oxide present. This analysis showed residual lead oxide contents less than 1.0 wt. percent PbO and in general less than 0.5 wt. percent PbO.

In the case of the blended mixtures particularly low residual lead oxide contents were found, being of the order of 0.2 wt. percent and less.

Tensile tests were performed on the extruded wires. The wires formed from the vibration mixed powders showed tensile strengths of the order of 3900 lb./in. The ball-milled powder mixtures gave rather scattered tensile strengths for different milling times of the mixtures but the strengths measured were all greater than 5000 lb./in. for powders milled for times of 1 /2 hours or more. Microscopic examination of the internal structure of these greater tensile strengths with increasing mixing times between 15 and 40 minutes, these tensile strengths varying from about 4500 to 5500 lb./in. Exceptionally and inexplicably a tensile strength of about 5450 lb./in. was found for wire extruded from the powder mixture blended for only 10 minutes. Further tests will be required to show whether or not this is a consistently reproducible result or only an unimportant freak result.

Of the three processes tried, ball milling and high speed blending gave higher strengths in the final products. The inferiority of the vibration mixed powders (although still giving a product stronger than ordinary lead may be in part due to a tendency for the lead and alumina powders to segregate during the vibration mixing due to their difference in density. Moreover vibration mixing is a gentler process and little mechanical deformation of the powder particles would be expected from it, whereas considerable deformation and forcing of alumina particles into the lead particles is produced by ball milling. High speed blending could also be expected to produce significant deformation due to impact of the rapidly rotating blades with the powder particles. Agglomerations of alumina would tend to be broken up by these two processes although any tendency for such agglomerations to form due to build up of electrostatic charges of the alumina particles would be minimised by the presence of the metallic lead. This would assist in dissipating such charges, especially in the case of the ball milling machine with its metal container as compared with the blender with its insulating glass container.

Microscopic examination showed that finer and more uniform dispersions were obtained with ball-milled powder mixtures than with blended mixtures. Moreover while similar tensile strengths were obtainable, the ballmilled mixtures in general gave better results in elongation measurements on the final product. Also, although shorter mixing times were required with high-speed blending, the need to perform this as a series of intermittent operations means that continuous supervision is required.

Of the three mixing processes, therefore, ball milling appears to be the most attractive. Using this process further experiments were performed to investigate the effect of different proportions of alumina in the lead powder. Mixtures containing 1, 3 and 5 vol. percent alumina powder were ball-milled for differing times according to their alumina content and then were reduced and extruded under the same conditions as before. The milling times increased progressively with the alumina content in order to ensure proper dispersion of the alumina among and within the lead particles. The extruded materials exhibited the physical characteristics given in the following table, which for comparison purposes also gives corresponding figures for materials extruded from the same powder without reduction of lead oxide.

1 /o A1203 ball-milled 3 /o A; ball-milled 5 /o A1 0; ball-milled 4 hours 5 hours 2 hours N 0!; Reduced Not Reduced Not Reduced Reduce Reduce educed Proof stress (lb./in.

0.0% elongation- 3, 660 3, 420 4, 280 4, 360 4, 560 5, 230 0.1% elon ation- 4, 350 3,860 5,080 4,940 5, 420 6,300 0.2% elongat1on 4, 900 4, 310 5, 670 5, 500 6, 7, 070 Tensile strength (Ill/UL 6, 030 5,120 7, 220 6, 340 7, 080 7, 950 Elongation percent on gauge length 4/A 7 14 1 10 0. 5 6 Reduction of area (percent)- 20 48 3 43 0 16 Youngs modulus X10 lb./in. 2. 1 1. 8 2. 2 2. 2 2. 6 3. 1 Extrusion pressure (ton/in!) at room temperature 20. 1 19. 1 27. 9 22. 0 30. 9 20. 3 Oxide content wt. percent PbO 3. 7 0. 20 8. 1 0. 52 12. 5 0. 44

specimens showed that, although some alumina agglomerations were present, the greater part of the alumina was uniformly dispersed and sufficiently fine to be only partially resolved at X600 magnification. 'For the blended Also for comparison purpose some samples of the lead powder containing no alumina were ball-milled for various times then subjected to the same lead oxide reduction treatpowder mixtures the extruded wires showed progressively 75 ment as before, being thereafter extruded in the same condition as before. The average tensile strengths obtained are given in the following table:

Time of ball milling, hours: Tensile strength, lb./in.

The small variations in tensile strength were consistent with variations in residual lead oxide content between samples.

Creep tests were conducted on samples of material made from a mixture of lead powder and 1% alumina by weight ball-milled reduced and extruded in accordance with the invention. This particular material had a tensile strength of 5500 lb./in. and elongation of 14%. Some of these creep tests were performed at room temperature (20 C.) and others at 80 C. For comparison, similar tests were performed on samples of pure lead at room temperature. The tests were stress-rupture tests, which consist in applying a fixed stress to a specimen at a fixed temperature and determining the time required under these conditions to bring about fracture of the specimen. The total elongation which had occurred was also measured after fracture. The results are given in the following table, in which a plus sign (-1-) indicates that at the time stated fracture had not occurred and the test was still continuing.

Time to fracture (hours) Stress Elongation (lb./in. Pia/1% A1203 Pb (Percent on 1 Minutes.

Heat treatment Density, gins/cc. after 1 hour temperature, C.

Pb/AlzOa (5%) Pb/PbO (3%) Room Temperature The strength of extruded lead-oxide-strengthened lead falls off appreciably as the extrusion temperature is increased, due to increasing size of the lead oxide particles produced by deformation at higher temperatures: with alumina, which is already in particulate form, the absence of this phenomenon permits higher extrusion temperatures, and therefore lower extrusion pressures, to be used without significant loss of strength in the final product.

This is demonstrated by the test results given in the following Table for alumina strengthened lead (Pb/A1 0 as compared with lead oxide strengthened lead (Pb/PbO). The respective contents of strengthening dispersoid were 3 vol. percent alumina and 7.5 vol. percent lead oxide. The relatively high proportion of lead oxide accounts for the higher strengths obtained at the lowest two extrusion temperatures.

In addition to providing a process for the production of dispersion strengthened lead, the present invention also includes a process for producing as a vendible product in its own right, a lead powder mixture suitable for the manufacture of dispersion strengthened lead by extrusion or other forming process preceded by chemical reduction, which powder production process consists in mixing lead powder and a refractory oxide powder in an oxidising atmosphere preferably in a ball-milling apparatusto produce an intimate mixture of the refractory oxide powder dispersed through the lead powder with the lead particles coated with lead oxide. It is contemplated that powder so prepared, being reasonably stable, may be kept for substantial periods before use for the production of dispersion strengthened lead by chemical reduction and subsequent forming.

The invention further contemplates within its scope lead oxide coated lead powder with a minor proportion of refractory oxide powder particles intimately dispersed therein in an amount preferably between 0.1 and 10 vol. percent inclusive, with at least a substantial proportion of the refractory oxide particles embedded in the lead particles, and also dispersion strengthened lead comprising a lead matrix with a substantially uniform dispersion of sub-micron alumina particles as the principal dispersion strengthening agent and a lead oxide content less than 1 wt. percent PbO and preferably less than 0.5 wt. percent, the alumina content being preferably between 0.1 and 10 vol. percent inclusive. For example, the lead may contain antimony in amounts as high as about 0.8%.

What 1 claim is:

1. A process for the production of dispersion strengthened lead comprising the step of intimately mixing lead powder and about 0.1 to 10% by volume of a refractory oxide powder in an oxidizing atmosphere so as to coat the lead particles with lead oxide and thereby inhibit agglomeration thereof by a pressure welding effect while obtaining a substantially uniform dispersion of the refractory oxide, subjecting the mixture to a reduction process for chemically reducing the lead oxide preferentially with respect to the refractory oxide by reason of the greater stability of the refractory oxide, and subjecting the so reduced powder mixture to a forming process to amalgamate the lead particles into a coherent matrix with the refractory oxide dispersed throughout.

2. A process as] claimed in claim 1 wherein the reduction process consists in subjecting the mixed powders to a temperature of about 300 C. in a stream of dry hydrogen for about 24 hours to give a residual lead oxide content of less than 1.0% PbO by weight.

3. A process as claimed in claim 1 wherein the forming process consists in extruding the reduced mixed powders.

4. A process for producing a lead powder mixture suitable for making dispersion strengthened lead by subsequent chemical reduction followed by forming, said process comprising the step of mixing lead powder and about 0.1 to 10% by volume of a refractory oxide powder in an oxidizing atmosphere to produce an intimate mixture of the refractory oxide powder dispersed through the lead powder with the lead powder particles coated with lead oxide.

5. The process claimed in claim 4 using alumina (A1 0 as the refractory oxide.

6. The process claimed in claim 5 using alumina powder of sub-micron particle size and lead powder having an average particle size of the order of 100 times greater than the alumina particles.

7. The process claimed in claim 5 using lead powder having a nominal particle size of less than 53 microns and -alumina powder of nominal particle size in the range 0.030 to 0.005 micron.

8. The process claimed in claim 4 using oxygen as the oxidizing atmosphere.

9. A process as claimed in claim 4 wherein the mixing step consists in ball milling the lead powder and the refractory oxide powder together.

10. A powder mixture for the production of dispersion strengthened lead by chemical reduction and subsequent forming of the powder, said powder mixture comprising lead oxide coated lead powder particles with about 0.1 to 10% by volume of refractory oxide powder particles intimately dispersed therein with refractory oxide particles embedded in the lead particles.

11. A powder mixture as claimed in claim 10 wherein said refractory oxide particles are alumina particles of submicron size.

References Cited UNITED STATES PATENTS 846,384 3/1907 Bailey 7555 2,985,571 5/1961 Binstook et al. 29-1825 3,044,867 7/1962 Edstrom 75-206 3,085,876 4/1963 Alexander et al. 29-4825 3,158,473 11/1964 Gatti 75-206 3,297,415 1/1967 Allen 29--182.5 3,315,342 4/1967 Roberts 75206 3,320,664 5/1967 Krantz et al. 75-20 CARL D. QUARFORTH, Primary Examiner.

-R. L. GRUDZIECKI, Assistant Examiner.

Us. 01. x11.